Method for producing composite materials

ABSTRACT

A method for continuously manufacturing in air high strength metal matrix composites reinforced with a plurality of parallel layers of unidirectional filaments comprising positioning a plurality of filament reinforced aluminum matrix monolayer tapes in a stack, pressing the stack between at least one pair of rollers heated to a predetermined temperature at pressures of approximately 100-10,000 psi to reduce its thickness and cause densification, said temperature being lower than the liquidus temperature of the aluminum matrix but being sufficiently high, in combination with the pressure, to cause bonding of the aluminum matrix of each tape to the filaments embedded therewithin and to contiguous portions of the adjacent tapes.

United States Patent Kreider et al.

METHOD FOR PRODUCING COMPOSITE MATERIALS Inventors: Kenneth G. Kreider,Glastonbury;

Karl M. Prewo, Manchester, both of Conn.

[73] Assignee: United Aircraft Corporation, East Hartford, Conn.

[22] Filed: Aug. 22, 1972 [21] Appl. No.: 282,818

[52] U.S. Cl 29/472.3, 29/419, 29/497.5, 29/498, 29/504 [51] Int. Cl823k 31/02 [58] Field of Search 29/472.3, 419, 498, 497.5, 29/504 [56]References Cited UNITED STATES PATENTS 2,468,206 4/1949 Keene et al29/472.3 X 3,419,952 1/1969 Carlson 3,436,816 4/1969 Lemelson 29/472.3 X

[ Mar. 5, 1974 3,551,996 1/1971 Sumner et a1 29/472.3 3 ,606,667 9/ 1971 Kreider 3,615,277 10/1971 Kreider et al. 29/472.3 X

Primary Examiner-Richard B. Lazarus Attorney, Agent, or Firm.1ohn D. DelPonti [5 7 ABSTRACT A method for continuously manufacturing in air highstrength metal matrix composites reinforced with a plurality of parallellayers of unidirectional filaments comprising positioning a plurality offilament reinforced aluminum matrix monolayer tapes in a stack, pressingthe stack between at least one pair of rollers heated to a predeterminedtemperature at pressures of approximately 10010,000 psi to reduce itsthickness and cause densification, said temperature being lower than theliquidus temperature of the aluminum matrix but being sufficiently high,in combination with the pressure, to cause bonding of the aluminummatrix of each'tape to the filaments embedded therewithin, and tocontiguous portions of the adjacent tapes.

5 Claims, 3 Drawing Figures METHOD FOR PRODUCING COMPOSITE MATERIALSBACKGROUND OF THE INVENTION Conventionally, the manufacture of highmodulus, high strength metal matrix composites in the aerospace industryis carried out in two major stages. In the first, monolayer tapes ofhigh modulus, high strength brittle filaments of boron, silicon carbideor silicon carbide coated boron are sandwiched between a metal foil anda plasma sprayed metal coating, the metal being aluminum, magnesium oralloys thereof, by a winding and plasma spray operation as disclosed,for example, in U.S. Pat. No. 3,606,667 assigned to the presentassignee. The second stage is the hot press diffusion bonding ofmultiple layers of these tapes to produce a multilayered composite.Typically, this stage requires the use ofa vacuum hot press capable ofproviding high pressures generally in the range of 2,000l0,000 psi for atime of 1 hour at elevated temperatures, usually 400-600 C. Thisconventional diffusion bonding procedure requires bonding pressures inexcess of 2,000 psi, a time cycle of several hours andan atmosphere ofvacuum or inert gas. The lengthy time requirement is related to thepractice of inserting the composite into the press prior to vacuumpump-down and heat-up and subsequent vacuum cooling after hot pressing.The heavy loading train plus dies generally require high powers and longtimes when they are enclosed in the vacuum system. The required time forheat-up bonding and cool-down to and from temperature, coupled with themagnitude of the pressures and temperatures needed for bonding cancause, in many cases, fiber degradation within the composite. Inaddition the need for pressures on the order of 10,000 psi can undulylimit the size of the composite to be manufacturered to the capacity ofthe hot press apparatus.

To overcome the limitations of diffusion bonding composites betweenfixed dies, the present invention, as described hereinafter, provides aninexpensive, continuous and flexible procedure for the production offully consolidated and bonded multilayered composites.

SUMMARY OF THE INVENTION The present invention relates to a process formanufacturing filament reinforced composites. More particularly, itrelates to a method for manufacturing high strength monolayer ormultilayer metal matrix composites reinforced with a plurality ofunidirectional filaments which can be done in air as a continuousprocess.

The present invention contemplates a roll bonding technique comprising,in a preferred embodiment, positioning a plurality of filamentreinforced aluminum tapes in a laminated stack, the tapes eachcomprising a. plurality of unidirectional filaments embedded in a plasmasprayed aluminum matrix, placing the lamination of tapes between atleast one pair of rollers preheated to a predetermined temperature,pressing the stacked tapes between the rollers at a pressure ofapproximately l-l0,000 psi, preferably 2005,000 psi, to reduce theirthickness and cause dens'ification, said temperature being lower thanthe liquidus temperature of the aluminum matrix but being sufficientlyhigh, in combination with the pressure, to cause bonding of the plasmasprayed metal matrix portion of each tape to the filaments embeddedtherewithin and to contiguous portions of adjacent tapes and removingthe densified composite. The inventive process is particularly suitedfor usage as an open air continuous process wherein successive portionsof the laminated tape are bonded and densified by continuous relativelinear motion between the composite workpiece and the rollers. Themethod is preferably performed either with the temperature maintainedbelow the solidus with a pressure of 2,000-l0,000 psi, preferably2,0005,000 psi, or at a temperature between the solidus and liquiduswith a pressure of approximately l002,000 psi, preferably SOD-2,000 psi.In either case, thickness reduction should be kept to not more than 50percent.

The inventive concept is applicable to various filaments, both flexible,such as stainless steel, steel, tungsten, molybdenum and titanium, andrelatively brittle, such as boron, silicon carbide, silicon carbidecoated boron, boron carbide, alumina and carbon which can be embedded byplasma spraying in an aluminum matrix. Unless otherwise stated, the wordaluminum is meant hereby to embrace both the elemental metal and itsalloys. In a preferred embodiment, relatively brittle high modulus, highstrength filaments such as the boron, silicon carbide and siliconcarbide coated boron filaments are embedded in the plasma sprayedaluminum which is adhered to a metal, preferably aluminum, foil.Although adjacent tapes may be laid up as cross plies at various angles,it is preferred that they be uniaxial and most preferably, with allfilaments oriented with their axes parallel to the axes of the rollers.

The inventive process is particularly directed toward a rapid continuousmethod for densifying and bonding plasma sprayed aluminum tapes. It hasbeen found that even though plasma sprayed aluminum is characterized bysurface oxide layers, the general porosity thereof, typicallyapproximately 15 percent, allows for a large amount of local deformationduring densification which acts to break up the surface oxide layersthat ordinarily prevent bonding. Such deformation exposes and bringsinto intimate bonding contact regions of virgin metal with the filamentsembedded therein as well as with adjacent mating surfaces. As will beappreciated by those skilled in the art, the present process centersonja relatively short time of exposure of the composite toelevatedtemperature so that densification and bonding can be achieved athigher temperatures without fiber degradation and also with lowerpressures so as to obviate the fiber crushing problem presently existingwith cross plied material and in complexly shaped parts. In general, thebonding procedure is performed in less than 40 minutes, preferably in2-15 minutes.

BRIEF DESCRIPTION OF THE DRAWINGS An understanding of the invention willbecome more apparent to those skilled in the art by reference to thefollowing detailed description when-viewed in light of the accompanyingdrawings wherein:

FIG. 1 is a cross-sectional viewof a monolayer composite tape having aplasma sprayed coating on one side only; 1

FIG. 2 is a cross-sectional view of a monolayer composite tape having aplasma sprayed coating on both sides thereof; and

FIG. 3 is a diagrammatic view, partly in section, showing the rollbonding of a plurality of monolayer composite tapes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT I While roll bonding,per se, is a known and commonly used method of diffusion bonding, theprocess is normally performed on parallel sheets of solid metal havingas a requirement, oxide-free surfaces which, when subjected toparticular elevated temperatures and pressures effect bondingtherebetween. Typically, large deformation, caused by a reduction inthickness of more than 50 percent during one rolling pass, is'used tobond oxide-free metals of Al, Mg, Cu, Pb or Zn with the operation beingperformed at room temperature.

When dealing with composites reinforced with filaments, however, suchlarge thickness reductions cannot be tolerated without experiencingfiber movement or degradation. It has been found that, in general,thickness reductions infilament reinforced plasma sprayed tapes must bekept to not more than 50 percentpOtherwise significant fiber damage isinflicted through fiber bending and/or by tensile loading of thefilaments by the shear stresses developed in the surrounding matrixduring deformation.

According to the invention multilayer fiber reinforced composites areprepared by rapidly heating a plurality of stacked filament reinforcedaluminum matrix tapes to the desired temperature. As shown in FIG. 1,preferred tapes 10 comprise unidirectional filaments l2 backed by analuminum foil sheet 14 and embedded in a plasma sprayed aluminum matrix16. In FIG. 2 is shown a tape identical with that of FIG. 1 except forthe additional plasma sprayed coating 18 on the opposite side of thefoil 14. As a further alternative, the foil sheet may be omitted and asimple tape comprising filaments embedded in the plasma sprayed matrixmay be utilized. The technique requires preheating of the componentsused in the bonding process so that the stacked tapes are heated to thepredetermined temperature and subjected to the selected pressure,generally ll0,000 psi,'for a period of time of less than 40 minutes andpreferably 2-15 minutes, during which time the tapes'are reduced inthickness by an amount of not more than 50 percent to become densified.

As shown in FIG. 3, a plurality of tapes are laminated, into a stack 20and passed between at least one roller pair 22. The tapes 10 are broughtto a preselected temperature during the roll pressing by suitableheating means such as electric resistance heating coils located in therollers themselves or by flame heating means, etc. The temperature ofthe workpiece during rolling must be lower than the liquidus temperatureof the matrix but be sufficiently high, in combination with saidpressure, to cause bonding of said matrix to said filaments and tocontiguous portions of adjacent tapes.

it has been found that by bringing the workpiece to temperature rapidly,i.e., in less than 5 minutes, preferably in 2 minutes, the need for avacuum, or even an inert atmosphere is obviated and the process can besuccessfully accomplished in air in an essentially continuous manner. I

The invention is illustrated by the following specific examples.

EXAMPLE I Filament reinforced monolayer tapes were prepared by theprocedure set forth in U.S. Pat. No. 3,606,667. A 1 mil sheet of 1,100aluminum foil was wound around a cylindrical mandrel. The foil was thenwound over with a continuous four mil filament of Borsic (boron with0.00010 to 0.00015 SiC coating) and plasma sprayed with 1,100 aluminumpowder on both sides. The plasma sprayed layer had a porosity ofapproximately 15 volume percent'. The resulting unconsolidated tape wasapproximately 16 mils thick and was cut into squares 3 inches on a side.From two to six of the squares were stacked with all of the filamentsbeing in the same direction and with the plasma sprayed surface beingmated with the foil at each interface. The deck thus formed was placedin a stainless steel foil envelope in order to hold all the pieces inplace during handling. The enveloped decks were heated in a furtime to600 C and passed between rolls of a rolling mill which had been heatedto 170 C. The heated decks were rolled with the fiber axis parallel tothe roll axis until they were fully densified by making one pass throughthe heated rolls, which took approximately 30 seconds. The resultingmultilayer composite possessed a thickness of 8 mils per layer, had avolume fraction of 24 percent filament,'with the plasma sprayed layersbeing fullydensified and bonded to those filaments which it surroundedand to theadjacent foil. The filaments retained a uniform distributionacross the cross section of the composite with no filament breakage.Specimens cut from the composites averaged 60,000

psi Ultimate Tensile Strength.

EXAMPLE II The same materials and conditions described in Example I wereused again except that the tapes utilized 6061 aluminum for both foiland spray, had a higher volume fraction of fiber and were plasma sprayedonly on one side. Each monolayer tape was approximately 10 mils thickprior to consolidation-The stacked monolayer tapes were heated in afurnace to 400 C and the resulting fully densified multilayer compositehad a 40 percent fiber volume fraction. The multilayer compositepossessed a thickness of 5 mils per layer. The fibers were uniformlydistributed and there was complete bonding as in Example I. The tensilestrength of specimens cut averaged out at 109,000 psi.

EXAMPLE m In this example, monolayer tape was made from 2 mil stainlesssteel wire, 606i aluminum foil and piasma sprayed 2024 aluminum powder.The stackedmonolayer tapes were heated to 600 C and the resultingdensified multilayer composite had a 24 percent fiber volume fractionwith uniform fiber distribution. There was complete bonding as inExamples 1 and II. The tensile strength averaged 101,000 psi.

In the practice of the present invention, it is recognized thatmodifications can be made and that other materials may be utilized.Other filaments, such as tungsten, molybdenum, titanium and steel, andother plasma sprayed metal matrices, such as magnesium, titanium, nickelor cobalt, may be utilized. It will readily be seen that through the useof the techniques hereinbefore described, multilayer fiber reinforcedcomposites of carefully controlled density can readily and reproduciblybe fabricated. While the invention has been described with reference tospecific examples, fabrication parameters and materials, theseembodiments and conditions are intended to be illustrative only. Variousmodifications and alternatives, other than those already mentioned willbe readily evident to those skilled in the art within the true spiritand scope of the invention as set forth in the appended claims.

What we claim is:

1. A method for manufacturing high strength metal matrix compositesreinforced with a plurality of parallel layers of relatively brittleunidirectional filaments comprising:

while in an air atmosphere positioning a plurality of filamentreinforced aluminum matrix tapes in a stack, each tape comprising aplurality of unidirectional relatively brittle filaments selected fromthe group consisting of boron, silicon carbide, silicon carbide coatedboron, boron carbide, alumina and carbon embedded in a plasma sprayedaluminum matrix;

placing said stacked tapes between at least one pair 6 of heatedrollers; rapidly bringing said stacked tapes to a temperature betweenthe solidus and liquidus of said aluminum matrix without the aid of aprotective atmosphere and roll pressing said stacked tapes between saidrollers at a pressure of IOU-2,000 psi to reduce their thickness andcause densification, said temperature being sufficiently high, incombination with said pressure, to bond said matrix to the filamentsembedded therewithin and to contiguous portions of adjacent tapes.

2. The process of claim 1 wherein the thickness of said tapes is reducedby not more than 50 percent.

3. The method of claim 1 wherein said filaments are selected from thegroup consisting of boron, silicon carbide and silicon carbide coatedboron.

4. The invention of claim 3 wherein said temperature is 200-600 C.

5. The invention of claim 4 wherein said filaments are oriented withtheir axes parallel to the axes of the rol-

2. The process of claim 1 wherein the thickness of said tapes is reducedby not more than 50 percent.
 3. The method of claim 1 wherein saidfilaments are selected from the group consisting of boron, siliconcarbide and silicon carbide coated boron.
 4. The invention of claim 3wherein said temperature is 200*-600* C.
 5. The invention of claim 4wherein said filaments are oriented with their axes parallel to the axesof the rollers.